1. Related Field
The invention relates to a sensor which consists of a housing defined by a measurement side and a connection side, which contains a coil placed on the measurement side of the housing. Therefore it concerns a contactless inductive displacement/distance sensor, like the one described in U.S. Pat. No. 3,473,110 from a long time ago.
2. Description of Related Art
Generic sensors are used these days in different areas with special requirement profiles, for example in aviation, railways, offshore areas, etc. Hermetically sealed inductive sensors are used in particular in aviation. These usually consist of a non-magnetic housing or a housing with very low magnetism, such as those made of stainless steel or titanium. The housing usually contains a core with ferromagnetic properties, for example made from “Permalloy” or from other ferrous sintered materials. The coil body is placed along with the core inside the housing. The measurement side of the sensor housing is typically finished with a lower material thickness. The connection side can be sealed with a glass-to-metal gasket, where typically a closing coupling, socket or plug is provided.
The sensor coil of the sensor described on U.S. Pat. No. 3,473,110 is embedded in a hard or ferrite pot core. This has a central cone, a floor and a thin external wall, which create a rotation-symmetrical opening in the form of a circular-shaped groove in the core, in which the coil is inserted. The hard pot core consists of sintered ferrite or soft magnetic steel, such as Permalloy. The housing consists of a non-ferromagnetic material in which the coil and the core are inserted. Stainless steel housings are typically used in the fields of aviation or hazardous industrial atmospheres. The housing has a locking cap which is either soldered or welded to the measurement side of the housing. The measurement side may also be closed, while the connection side may have a hole for inserting the coil and the core. Either an integrated cable, a plug, or a coupling/socket are intended for the electrical connection.
The sensor described on U.S. Pat. No. 3,473,110 is problematic in many ways. Thus it is extremely difficult to create a permanent seal between the cable and the housing. The sealing required between the grout and the housing in order to achieve a hermetic seal is hard to accomplish. This is caused by common conditions such as high temperature, temperature variations, aggressive media, high or low pressure, alternating pressure, etc. In particular, a high number of pressure or temperature changes, like the ones which arise on airplanes, are the ones which may cause leaks and/or a deficient seal.
There are also sensors with integrated connectors instead of built-in plugs/sockets, instead of integrated cables. Waterproof plugs are extremely costly and expensive to produce. If the measurement side of the sensor housing is sealed, it has shifted the problem of achieving an adequate seal to the connection side. Waterproof plug connectors have the disadvantage that they are expensive. An integrated plug and/or a corresponding coupling/socket also requires a significant space, which results in an increased sensor size. This applies especially as the mating plug increases even more the required installation space of the sensor. In comparison with a simple connection over a built-in cable, the required space can almost triple.
In the known sensor, it is also very expensive to drill or mill the sensor housing from the connection side, especially since the wall thickness on the measurement page should be very thin in order to affect the electromagnetic fields as little as possible.
Finally, significant costs arise as a result of the required core: namely, on the one hand, the cost of the core, and, on the other, costs caused by the assembly with the core. Cores made from Permalloy or similar “magnetically soft” metals must be machined, making them costly and expensive. Furthermore, they must be inserted in the housing through complex operations: for example, a spring element placed on the connection side presses them against the measurement side, in order to bring the core and the coil to a defined and temperature-stable fixed position towards the front and/or the measurement side of the housing.